1. Field of the Invention
The present invention relates to a lens used in an optical recording/reproducing apparatus such as an optical disk system or an optomagnetic disk system and to an optical recording/reproducing apparatus using such a lens.
2. Description of the Prior Art
In an optical disk system as shown in FIG. 1, a beam 2 emitted from a light source 1 is transmitted through a beam splitter 3 and is collimated into a parallel beam by a collimator lens 4. The beam 2 is then focused onto a recording surface 6A of an optical disk 6 by an objective lens 5.
The beam 2 reflected by the recording surface 6A is converted into a parallel beam by the objective lens 6 and becomes incident on a photodetector 7 through the collimator lens 4 and the beam splitter 3.
When tracking control in such an optical disk system is performed by a 3-spot method, the beam 2 emitted from the light source 1 is split into three beams by a diffraction grating 8, and the three beams are focused onto the recording surface 6A.
In this case, two auxiliary beams among the three beams become incident on positions of the objective lens 5 other than its optical axis. Therefore, unless monochromatic aberration including not only spherical aberration but also coma and so on are sufficiently corrected, high-density recording/reproduction cannot be performed.
Even if a single beam 2 is used, unless the coma and so on of the objective lens 5 are sufficiently corrected, extremely high precision is required in assembly of the apparatus, resulting in an expensive apparatus.
In order to improve the response characteristics of an optical pickup to focusing and tracking control, the objective lens 5 must be compact in size and light in weight.
As an example of such an objective lens 5, a compound lens of 2-group 3-lens configuration and of a general optical glass having a uniform refractive index is widely used, as described in Japanese Patent Disclosure No. 55-4068.
However, this type of lens has a large number of surfaces (6 in all) to be polished, and requires assembly and centering of respective lens components. Thus, such a lens requires a large number of steps and manufacturing cost cannot be reduced. If a plurality of lens groups are used as in this type of lens, a lens barrel must be used, additionally preventing a light and compact lens.
In order to satisfy demand for a light and compact lens, a single nonspherical lens has been proposed which consists of a plastic material having a uniform refractive index, as described in Japanese Patent Disclosure No. 50-156945.
This lens can be manufactured at low cost by utilizing a molding technique. However, since the medium is a plastic material, the lens is subject to problems of low resistance to humidity and heat. The lens is also easily scratched and provides only low reliability. An aspherical lens consisting of glass is extremely expensive to polish and has not been used in practice to date.
A graded index lens has been proposed as a solution to problems with a lens consisting of a medium having a uniform refractive index. For example, U.S. Pat. No. 3,729,253 describes a single spherical lens which has a refractive index gradient along the radial direction so that the spherical aberration, coma and astigmatism of 3rd order are reduced.
However, in this lens, spherical aberration of high order, i.e., 5th or higher is not considered, though the lens is in fact subject to additional spherical aberration of a high order. For this reason, imaging characteristics on the lens optical axis are poor, and the lens is not suitable for an optical disk system or the like which requires a high numerical aperture.
A lens for an optical disk system or the like must have a substantially zero spherical aberration of each order, must be able to focus the diffraction spot on the axis to a degree equivalent to the diffraction limit, and must have an rms value of wavefront aberration of about 0.07.lambda. or less.
Japanese Patent Disclosure No. 55-6354 describes a design of a lens which can reduce spherical aberration and which is for use in a video disk.
In such a lens, only correction for spherical aberration is performed, and the sine condition representing the coma is not satisfied. In an embodiment of this lens, since a radius of curvature R.sub.2 of a second surface is negative and the second surface has a convex shape, the deficiency amount of the Abbe's sine condition is increased with a decrease in .vertline.R.sub.2 .vertline.. The field of view is also limited due to coma. For these reasons, this lens is not practical.
A graded index lens in which both the spherical aberration and coma are corrected is described in Japanese Patent Disclosure No. 58-122512. However, a second surface of this lens has a concave shape wherein a radius of curvature C.sub.1 satisfies 0.09 .ltoreq.C.sub.1 .ltoreq.0.14 and the lens has a meniscus shape. Since polishing of the two surfaces in alignment with each other is not easy, the cost of the lens cannot be decreased. The lens also has a central thickness t within a range between 8 and 15 mm, and is heavier than a conventional 2-group 3-lens component lens. Thus, the advantages of light weight and compactness of a single lens are impaired.
In this manner, conventional lenses cannot satisfy requirements in a lens for an optical recording/reproducing apparatus to a degree where at least spherical aberration and coma are sufficiently corrected to high orders, the lens is compact in size, light in weight, and inexpensive.
For this reason, an optical recording/reproducing apparatus using such a lens cannot perform high-density recording/reproduction, extremely high precision in assembly of the apparatus is required, and response characteristics of an optical pickup to focusing and tracking control are poor.